1. Field of the Invention
The present invention relates to a method of formation of a magnetostrictive layer and to a strain sensor, wherein a magnetostrictive layer is formed for use for detection of strain in torque sensors, strain sensors, etc. in automobiles, robots, electrically powered machinery, etc.
2. Description of the Related Art
In the control of automobiles, robots, electrically powered machinery, etc., the accurate, quick detection of the torque of the rotational drive system and the stress-strain of different members is important. As a method for detecting the strain caused in substances due to such force, pressure, torque, etc., consideration has been given to sensors which use the magnetostrictive effect of amorphous magnetic alloys. As the features of such sensors, mention may be made of possible noncontact detection, possible simple structure and small size, and high sensitivity.
In construction, a strain sensor using the magnetostrictive effect of amorphous magnetic alloys is comprised of a detected member having on its surface a magnetostrictive layer comprised of an amorphous alloy and a magnetic sensing portion disposed a certain distance from the same.
As the method for formation of the amorphous alloy layer, consideration has been made of the method of adhering an amorphous alloy ribbon on the surface of the detected member by a resin and the method of imparting shape magnetic anisotropy at that time, but there are problems in the bonding strength, such as peeling, and the characteristics decline as the temperature rises, so the reliability is poor. Further, when an amorphous alloy ribbon is bonded by welding or soldering, the bonding strength is improved compared with adhesion by a resin, but the amorphous alloy layer ends up crystallizing and therefore the magnetic characteristics end up greatly degraded. The method of formation of an amorphous alloy layer by the sputtering method or other vapor deposition methods increases the manufacturing costs due to the slow speed of film formation and the large facilities needed and also there are problems in the bonding strength.
Therefore, the method has been devised of forming an alloy layer with a high glass-forming ability on the surface of the detected member by the hot isostatic press (HIP) or other method, then irradiating a high energy density beam in a stripe manner to leave alternate unirradiated portions, make partially amorphous only the irradiated portions on the surface by the melting and rapid quenching accompanying this, and thereby impart shape magnetic anisotropy (for example, see Japanese Unexamined Patent Publication (Kokai) No. 63-280476 and Japanese Unexamined Patent Publication (Kokai) No. 3-160337).
In the above method, however, problems ended up occurring such as the decline in strength due to the nonuniformity accompanying the difference in the mechanical properties of the irradiated portions and the unirradiated portions and the fact that since magnetic flux mainly flows only at the irradiated portions, while shape magnetic anisotropy is induced, the sensor output (sensitivity) is not improved that much.
Further, a method of irradiating a high energy density beam on a cold paramagnetic steel material to form a magnetostrictive layer by the accompanying surface reformation is described in Japanese Unexamined Patent Publication (Kokai) No. 1-209773, but it is described in that publication that the irradiation should be performed so that adjoining beam-treated lines should not overlap over half of their widths. A magnetostrictive layer obtained by reforming such a cold paramagnetic steel, however, is considerably inferior in magnetic permeability and other soft magnetic characteristics compared with a magnetostrictive layer comprised of an amorphous alloy, so a high sensor sensitivity cannot be obtained. Further, the sensor sensitivity does not necessarily become high when the overlap between beam-treated lines is less than half of their widths.
In the above way, in the past, consideration has been given to a magnetostrictive layer including an amorphous phase having a stripe-like structure of irradiated portions and unirradiated portions of a high energy density beam arranged alternately and a magnetostrictive layer obtained by surface reformation of a cold paramagnetic steel with the overlap of the beam-treated lines made less than half the width. In these cases, however, there were problems of a low strength of the magnetostrictive layer and a low sensor sensitivity.